Geologic
Time and
Earth
History
Two Conceptions of Earth History:
Catastrophism
• Assumption: Great Effects Require Great Causes
• Earth History Dominated by Violent Events
Uniformitarianism
• Assumption: We Can Use Cause And Effect to
Determine Causes of Past Events
• Finding: Earth History Dominated by Small-scale
Events Typical of the Present.
• Catastrophes Do Happen But Are Uncommon
Two Kinds of Ages
Relative - Know Order of Events But Not
Dates
• Civil War Happened Before W.W.II
• Bedrock in Wisconsin Formed Before The
Glaciers Came
Absolute - Know Dates
• Civil War 1861-1865
• World War II 1939-1945
• Glaciers Left Wisconsin About 11,000 Years
Ago
Absolute Ages: Early Attempts
The Bible
• Add up Dates in Bible
• Get an Age of 4000-6000 B.C. For Earth
• John Lightfoot and Bishop Ussher - 4004
B.C. (1584)
• Too Short
Absolute Ages: Early Attempts
Salt in Ocean
• Rivers bring dissolved solids to ocean
• If we know rate salt is added, and how
much salt is in ocean, can find age of
oceans.
• Gave age of about 100 million years.
• Problems
– Is rate at which salt is added constant?
– How much salt leaves ocean?
Absolute Ages: Early Attempts
Sediment Thickness
• Add up Thickest sediments for each period
• Estimate rate of deposition to find age
• Problem: rates of deposition very variable!
• Indicated ages of at least 100 million years
Age of The Sun
• If sun gets its heat from burning or other chemical
reactions, could only last 10,000 years or so.
• Best 19th century guess: sun was slowly
contracting.
• Problem: only 30 million years ago, sun would
have extended out to earth's orbit!
• Geologists wanted more time, but you can't fight
the laws of physics...
• Sun actually gets its energy from nuclear
reactions and can keep going for billions of years
• The Geologists were right after all. Go Team.
Radiometric Dating: Half-Life
How Do We Measure Half-Lives
of Billions of Years?
• Number of Decays/Unit Time =
k * Number of Atoms
• k = Decay Constant
• Leads to N = N0exp(-kt)
(This is what the exp button on your calculator
is for)
• Half-Life = 0.693/k
Present Radiometric Dating Methods
Cosmogenic
• C-14 5700 Yr.
• Be-10 2.5 M.Y.
Primordial
• K-Ar (K-40) 1.25 B.Y.
• Rb-Sr (Rb-87) 48.8 by
• U-235 704 M.Y.
• Th-232 14 B.Y.
• U-238 4.5 B.Y.
Primordial
• Nd-Sm (Sm-147-Nd-143)
106 B.Y.
• Re-187 43 B.Y.
• Lu-Hf (Lu-176) 36 B.Y
• Fission
The
Geologic
Time Scale